Fathead Minnow Populations Research Articles

Description, life cycle, and development of the myxozoan Myxobolus rasmusseni n. sp. in fathead minnows, Pimephales promelas: A possible emerging pathogen in southern Alberta, Canada

Morphological, gene sequence, host tissue tropism, and life cycle characteristics were utilized to describe the myxozoan, Myxobolus rasmusseni n. sp. from fathead minnow, Pimephales promelas, collected from reservoirs in southern Alberta. Results from serial histological sections of whole heads showed that myxospores were contained within irregular-shaped and sized coelozoic capsules (=plasmodia). Clusters of membrane-bound, myxospore-filled plasmodia filled the head cavities of juvenile fathead minnows, leading to the development of large, white, disfiguring lesions in mid to late summer. Bilateral exopthalmia (pop-eye disease) was a common outcome of M. rasmusseni n. sp. development. BLASTn search of a 1974 bp sequence of the 18S rDNA gene isolated from myxospores indicated that M. rasmusseni n. sp. was distinct from other coelozoic and histozoic Myxobolus spp. cataloged in GenBank. 18S rDNA gene sequences from triactinomyxon spores released from the oligochaete Tubifex were 100% identical to sequences from myxospores collected from syntopic fathead minnows. Results from a longitudinal survey of the 2020 cohort of fathead minnows showed that young-of-the-year are exposed at 1–5 mo and that 60–90% of these had developed myxospore-filled lesions approximately one year later. Data regarding potential sources and timing of M. rasmusseni n. sp. emergence in fathead minnow populations are needed.

Comparative Effects of Embryonic Metformin Exposure on Wild and Laboratory-Spawned Fathead Minnow (Pimephales promelas) Populations.

Metformin is routinely detected in aquatic ecosystems because of its widespread use as a treatment for Type 2 diabetes. Laboratory studies have shown that exposure to environmentally relevant concentrations of metformin can alter metabolic pathways and impact the growth of early life stage (ELS) fish; however, it is unknown whether these effects occur in wild populations. Herein, we evaluate whether findings from laboratory studies are representative and describe the relative sensitivities of both populations. Duplicate exposures (0, 5, or 50 μg/L metformin) were conducted using wild- and lab-spawned fathead minnow (Pimephales promelas) embryos. Apart from the water source, exposure conditions remained constant. Wild embryos were exposed to previously dosed lake water to account for changes in bioavailability, while reconstituted freshwater was used for the laboratory study. Developmental metformin exposure differentially impacted the growth and morphology of both cohorts, with energy dyshomeostasis and visual effects indicated. The fitness of wild-spawned larvae was impacted to a greater extent relative to lab-spawned fish. Moreover, baseline data reveal important morphological differences between wild- and lab-spawned ELS fatheads that may diminish representativeness of lab studies. Findings also confirm the bioavailability of metformin in naturally occurring systems and suggest current exposure scenarios may be sufficient to negatively impact developing fish.

Health status of fathead minnow (Pimephales promelas) populations in a municipal wastewater effluent-dominated stream in the Canadian prairies, Wascana Creek, Saskatchewan.

Their unique hydrological and climatic conditions render surface water systems in the southern Canadian Prairies at an elevated risk from exposure to contaminants released from municipal wastewater effluents (MWWEs). The aim of this study was to characterize the potential health effects and their underlying molecular mechanisms in populations of fathead minnow (Pimephales promelas; FHM) in Wascana Creek, an effluent dominated stream in Southern Saskatchewan, Canada. Studies were conducted during the spawning season in 2014 and 2015 to assess responses in terms of overall health, reproductive functions, plasma sex steroid hormone levels, and expression of selected genes along the hypothalamus-pituitary-gonadal axis. FHM downstream of the effluent fallout had lower gonadosomatic indices and significantly greater hepatosomatic indices compared to upstream populations. In both male and female FHMs, significantly greater occurrence and severity of gonadal degradation and delayed maturation were observed in downstream fish compared to upstream fish. Downstream males also displayed lower scores of secondary sexual characteristics and a decreasing trend in plasma 11-ketotestosterone levels. Interestingly, no indications of exposure to estrogenic compounds, such as occurrence of testicular oocytes were observed, which was in accordance with the lack of presence of key biomarkers of estrogenic exposure, such as induction of vitellogenin. In general, expression of the majority of transcripts measured in FHMs downstream of the effluent fallout was significantly downregulated, which supports observations of the general deterioration of the health and reproductive status of these fish. Chemical analysis indicated that 10 pharmaceuticals and personal care products (PPCPs) were present at the downstream site, some at sufficiently great concentrations that may present a risk to aquatic organisms. With continuous exposure to a diverse number of stressors including high nutrient and ammonia levels, the presence of a variety of PPCPs and other contaminants, Wascana Creek should be considered as an ecosystem at risk.

Endocrine disruption: From a whole-lake experiment to a calibrated ecosystem model

The unique Canadian whole-ecosystem study on the impact of 17α-ethinylestradiol (EE2) on a freshwater food web (Karen et al., 2014) provides evidence of the value of whole-ecosystem experiments for understanding indirect effects of endocrine disrupting compounds (EDCs) and other aquatic stressors. To further explain the indirect effects of EE2 observed in the experimental lake, an ecosystem model based on AQUATOX equations was successfully developed and calibrated. Discussions with the scientists who gathered the experimental data were necessary to ensure the consistency of the parameters used in the model and the realism of the biomass dynamics observed in such ecosystems. The prediction results helped further explain how the other fishes were impacted by the fathead minnow collapse. This study also suggests that a mix of reduced gamete production, increased gamete mortality and fish mortality is a potential mechanism for the collapse of the fathead minnow population.

Abiotic Controls of Invasive Nonnative Fishes in the Little Colorado River, Arizona

We examined why the lower Little Colorado River (LCR), Arizona continues to be dominated by four native fish species despite decades of encroachment by nine nonnative species. Most perennial flow begins at Blue Spring, located at river kilometer (RKM) 20.74 above the LCR's mouth, which is supplemented downriver by other springs. Blue Spring's water is extremely supersaturated with dissolved CO2 (658 mg/L), and at baseflow the CO2 concentrations apparently functioned as a fish-barrier for about 1 km further downriver until it outgassed to 288 mg/L. Thereafter, CO2 outgassed to 202 mg/L by the top of Chute Falls (RKM 14.20) and 76 mg/L near the river's mouth. Chute Falls, and high CO2 levels above it, obstruct upriver, but not downriver, fish movements; however, we still captured fishes from eight nonnative species above this falls. Our findings indicate most warmwater nonnative fishes in the LCR were immigrants from upriver sources within and bordering the intermittent corridor that invaded the system during floods, while most coldwater trout invaded from the Colorado River. Most nonnative species failed or were severely limited at reproducing progeny in the LCR, except occasionally by channel catfish Ictalurus punctatus and fathead minnow Pimephales promelas in the lower 13.57 km corridor, which was likely related to CO2. Correlation tests suggested severe flood and/or suspended sediment regimes reduced populations of fathead minnows, red shiners Cyprinella lutrensis, and plains killifish Fundulus zebrinus in the LCR. Conversely, severe suspended sediment regimes (especially ≥60,000 mg/L) increased channel catfish, black bullhead Ameiurus melas, and common carp Cyprinus carpio, hypothetically by paralyzing these fishes upriver, whereby many drifted downstream and recovered in the lower LCR; however, their abundances decreased after prolonged periods of high CO2. Nonnative fishes are controlled in the LCR by the system's endemic abiotic conditions.

The Effect of Synthetic Estrogens on Fathead Minnow Populations

The fathead minnow (Pimephales promelas) is a freshwater fish with a wide distribution in lakes, streams, and ponds across North America, including Ontario. This species has been recognized as one of the most useful model organisms for freshwater toxicology monitoring, due to its tolerance of its range for aquatic conditions. Synthetic estrogens are excreted as a metabolic product in the urinary waste of women using oral contraceptives. Estrogens are not effectively filtered out by sewage treatment plants and are subsequently released into water effluents, affecting aquatic wildlife. In response to experimentally heightened estrogen concentrations, male and female fathead minnows have shown increases in vitellogenin protein, and females have displayed delayed ovarian development and increased numbers of underdeveloped ovarian follicles. Males also experienced lowered gonadosmatic indices, and some males had primary stage oocytes in their testes. As would be expected, synthetic estrogen additions influenced fathead minnow reproductive success, leading to a collapse of the experimental fish population. By taking a multi-disciplinary approach, this study looks to identify how synthetic estrogens are threatening local fish populations, and how this may eventually disrupt the freshwater food chain. Collaboration with The Canadian Association on Water Quality (Kingston), Ravensview Wastewater Treatment Plant, and the Catarqui Region Conservation Authority will provide a greater understanding of the hormonal content in Kingston’s wastewaters. Ultimately, this study will provide more insight on the level of risk faced by freshwater fish populations on a local scale, and propose a method to reduce estrogen deposition in freshwater environments.

Field verification of chondroitin sulfate as a putative component of chemical alarm cue in wild populations of fathead minnows (Pimephales promelas)

Chemical characterization of damage-released alarm cues in ostariophysan fishes has lagged far behind the study of the ecological role that these cues play in behavioral decision-making of prey fishes. Chondroitin sulfate has been identified as a putative component of alarm cue based on two laboratory studies of zebrafish, Danio rerio, and the northern studfish, Fundulus catenatus. The fathead minnow, Pimephales promelas, is a model organism in the study of chemically mediated predator–prey interactions, in part because they can be studied in the laboratory and under field conditions. Here, we conducted a field experiment on wild populations of fathead minnows, to test for area avoidance of chondroitin sulfate relative to conspecific skin extract (containing alarm cues = positive control) or water (negative control). We repeated the experiment in two small lakes in central Minnesota using minnow traps containing blocks of sponge with one of the three test cues. We found that fathead minnows avoided traps chemically labeled with conspecific alarm cue more than control traps labeled with water, and that the number of minnows caught in traps labeled with chondroitin sulfate was intermediate between alarm cue traps and water traps. These data are consistent with laboratory findings that chondroitin sulfate is a component of alarm cue, but that other species-specific compounds are needed for a full behavioral response.

Expansion of the Known Host Range of the Microsporidium, Pseudoloma neurophilia.

The microsporidium, Pseudoloma neurophilia, is the most common infectious organism found in laboratory zebrafish colonies. Many currently used zebrafish lines originally came from pet store fish, and the initial description of P. neurophilia came from zebrafish obtained from a retail pet store. However, as P. neurophilia has not been described from wild-caught zebrafish, whether P. neurophilia is a natural pathogen of zebrafish is an open question. The pooling of fish of different species in the aquarium fish trade is common and a generalist parasite could be transmitted to novel hosts in this scenario. We determined that P. neurophilia can infect seven species of fishes from five families by cohabitation with infected zebrafish: Betta splendens, Xiphophorus maculatus, Devario aequipinnatus, Pimephales promelas, Oryzias latipes, Carassius auratus and Paracheirodon innesi. Infections in these fishes were histologically similar to those of zebrafish. We include a case report of a laboratory population of fathead minnows with naturally acquired P. neurophilia infections. With such a broad host range, including several fish families, other laboratory fishes should be screened routinely for this and other microsporidian parasites.

Recovery of a wild fish population from whole-lake additions of a synthetic estrogen.

Despite widespread recognition that municipal wastewaters contain natural and synthetic estrogens, which interfere with development and reproduction of fishes in freshwaters worldwide, there are limited data on the extent to which natural populations of fish can recover from exposure to these compounds. We conducted whole-lake additions of an active component of the birth control pill (17α-ethynylestradiol; EE2) that resulted in the collapse of the fathead minnow (Pimephales promelas) population. Here we quantify physiological, population, and genetic characteristics of this population over the 7 years after EE2 additions stopped to determine if complete recovery was possible. By 3 years post-treatment, whole-body vitellogenin concentrations in male fathead minnow had returned to baseline, and testicular abnormalities were absent. In the spring of the fourth year, adult size-frequency distribution and abundance had returned to pretreatment levels. Microsatellite analyses clearly showed that postrecovery fish were descendants of the original EE2-treated population. Results from this whole-lake experiment demonstrate that fish can recover from EE2 exposure at the biochemical through population levels, although the timelines to do so are long for multigenerational exposures. These results suggest that wastewater treatment facilities that reduce discharges of estrogens and their mimics can improve the health of resident fish populations in their receiving environments.

Reproductive and health assessment of fathead minnows (Pimephales promelas) inhabiting a pond containing oil sands process-affected water

Previous laboratory based studies have shown that oil sands process-affected waters (OSPWs) containing high concentrations of naphthenic acids (>25mg/l) have adverse effects on the reproductive physiology of fish. The purpose of this study was to assess the reproductive development and health of a wild population of fathead minnows (Pimephales promelas) inhabiting an OSPW pond that has moderate concentrations of naphthenic acids (∼10mg/l). Fathead minnows were collected at various times during the period of 2006 through 2008 from Demonstration Pond (OSPW) located at Syncrude Canada Ltd., and two reference sites, Beaver Creek reservoir and Poplar Creek reservoir, which are all north of Fort McMurray, AB, Canada. Condition factor, gill histopathology, gonadosomatic indices, liver somatic indices, male secondary sexual characteristics, and plasma sex steroids were examined. Depending on the time of year that fathead minnows were collected, there were differences in the condition factor, gonadosomatic indices, liver somatic indices, and secondary sexual characteristics of fathead minnows (in males) from Demonstration Pond when compared to the fathead minnows from the reference sites. In comparison to reference fish, lower concentrations of 11-ketotestosterone were measured in the plasma of male fathead minnows collected from Demonstration Pond in June 2006 and July 2007. Black spot disease and Ligula intestinalis were prevalent in fathead minnows from the reference sites but were not observed in fathead minnows from Demonstration Pond. The opercula of fathead minnows from Demonstration Pond also differed from those of reference fish. An examination of the gills of fathead minnows from Demonstration Pond revealed that were a number of proliferative and degenerative alterations relative to reference fish. Even though the fathead minnow population has been maintained in this OSPW pond since 1993, the results of this study demonstrate that the OSPW continues to affect the reproductive development and health of the fathead minnows compared to fish collected at reference sites.

Metallothionein and Heat-Shock Protein 70 Induction in Caged and Wild Fathead Minnows (Pimephales promelas) Exposed to the Ouachita River, Louisiana

The purpose of this study was to explore the changes in mRNA expression levels for metallothionein subtype 2 (MT-2) and heat-shock protein 70 (HSP70) in fathead minnows in response to environmental exposure in a mercury (Hg)-contaminated freshwater ecosystem. It was hypothesized that expression levels of both genes may rise concurrent with the bioaccumulation of Hg and possibly other heavy metals during exposure to the Ouachita River. The experimental design incorporated three distinct populations of fathead minnows: (1) a negative control population of laboratory-raised fathead minnows unexposed to heavy metals or other contaminants, (2) laboratory-raised fatheads placed in cages and exposed to a contaminated ecosystem for 2 wk, and (3) wild-caught (native) fathead minnows captured at the same site where caged fatheads tested positive for Hg bioaccumulation. Study endpoints included growth rates and gross pathology at necropsy. Total Hg levels of the water at the exposure sites as well as in whole fish homogenates were determined by cold vapor atomic absorption spectroscopy (AAS). AAS was also used to assay levels of lead (Pb) and copper (Cu), though these were below detectable limits. Hepatic expression levels of MT and HSP70 mRNA were determined by quantitative real-time reverse-transcription polymerase chain reaction (qRT-PCR). As hypothesized, levels of both transcripts were significantly increased in the caged exposure group and native fish group compared to unexposed control fish. In addition, the native fish group had significantly higher levels of expression for both genes when compared to caged exposed fish.

Plasticity of diel patterns in the diet and habitat use of feral, non-native fathead minnow Pimephales promelas (Actinopterygii, Cyprinidae) in a pond-dwelling population in England

The aim of the study was to test for diel patterns in the diet and habitat use in a feral, pond-dwelling, non-native fathead minnow Pimephales promelas population in England. Fish were collected in June 2009 using traps set in four habitat types (open waters, rushes—Juncuseffusus, weeds—Potamogeton natans and mixed vegetation), then sexed and measured for total length and eviscerated weight in the laboratory. Data were analysed at 6-h intervals, with Fulton’s condition index and three dietary parameters (frequency of occurrence, number and weight) calculated. Minor diurnal differences in habitat use were observed in males, females and immatures, and these may be due to predator avoidance. Body condition varied greatly in rushes during daytime, probably due to shifts in habitat suitability (e.g. food, refuge). Detritus dominates the diet of native fathead minnow; however, planktonic crustaceans were the most important food resource for this population, with a clear ontogenetic shift, irrespective of habitat, towards greater proportion of ingested detritus in larger individuals. Overall, the results demonstrate that feral fathead minnows display substantial trophic plasticity and a wide range of habitat use, which is normally associated with invasive species. However, established fathead minnow populations in Europe are rare despite its wide-spread ornamental and scientific use.

Time-specific and population-level differences in physiological responses of fathead minnows (Pimephales promelas) and golden shiners (Notemigonus crysoleucas) exposed to copper

The influence of exposure time on gill Na+/K+ ATPase activity and metabolic rate in populations of fathead minnows (Pimephales promelas) and golden shiners (Notemigonus crysoleucas) hatcheries in Ohio (OH) and Pennsylvania (PA) when exposed to sublethal concentrations of copper (Cu) was examined. The pattern of change in gill Na+/K+ ATPase activity was similar in all species/populations and results support expectations based on the concept of acclimation. In all populations, Na+/K+ ATPase activity declined significantly compared to reference values within 24h, recovered by 48h, and then continued to increase before exceeding reference values by 192h. With the exception of PA fathead minnows, Na+/K+ ATPase activities returned to reference levels by 384h. Although metabolic rates of individual fish were not strongly correlated with Na+/K+ ATPase activities, the pattern of change in mean values of these physiological parameters was very similar. However, OH populations of both fathead minnows and golden shiners demonstrated much more dramatic changes in metabolic rate compared to PA fish. At 24h, metabolic rate of PA fathead minnows had decreased by 16% compared to the reference value whereas the OH population had decreased by 31%; metabolic rate of PA golden shiners declined by 23% compared to 59% in OH shiners at 24h. Similar differences were observed in the maximum metabolic rates achieved at 192h. While the increased sensitivity of OH fish to Cu is not readily explainable by genetic or environmental factors, results suggest the need for considering population level differences when evaluating the physiological effects of toxicants.

Alarm substances elicit limited population‐level responses in fathead minnow

Abstract – Alarm substances, chemical cues released by injured prey, are associated with increased predation risk. In the laboratory, fathead minnows limit conspicuous behaviours, such as foraging and nest guarding, when exposed to these cues. Although such responses could lead to reductions in growth and reproduction, with subsequent demographic consequences (e.g., recruitment), effects of alarm substances at the population level are largely unknown. In three field experiments, ponds and cattle troughs stocked with fathead minnow populations were treated with alarm substances or a water control over the summer breeding and growing season. Alarm substances had no effect on male parental behaviour. In one experiment, spawning occurred earlier with exposure to alarm substances; however, there was no subsequent effect on recruitment of young in this or in the other two experiments. Despite individual‐level effects in short‐term experiments, repeated exposure to alarm substances over a season had no measureable impact at the population scale.

Effects of nest substrate on egg deposition and incubation conditions in a natural population of fathead minnows (Pimephales promelas)

Fathead minnows ( Pimephales promelas Rafinesque, 1820) occur commonly in small lakes across the prairie region of North America. Males establish nests, court females, and tend the eggs. In our study population in Minnesota, males choose between two distinct nest substrates that differ in exposure to abiotic conditions affecting egg development: the underside of pond-lily (genus Nuphar Sm.) pads or the underside of submerged sticks. We found that lily pad nests contained more eggs, more clutches of different ages, and eggs distributed over a larger area than nests under submerged sticks. The difference in egg incubation temperatures between lily pad nests and stick nests varied diurnally with air–water temperature gradients. Proximity to the surface allowed lily pad nests to be significantly warmer during the day but only slightly cooler at night than nests under submerged sticks, resulting in about 10 h shorter time to hatch. The availability of lily pads for nests may greatly alter selection on sexual behaviour by males in competing for limiting nest sites of high quality, and female opportunity to exercise choice of mate. Moreover, availability of lily pads changes the role of abiotic conditions. In warm weather, males that nest under lily pads may achieve greater reproductive success than males that nest under sticks.

Consumption estimates of walleye stocked as fry to suppress fathead minnow populations in west‐central Minnesota wetlands

Abstract – Fisheries managers throughout the Prairie Pothole Region of Minnesota often use semi‐permanent and permanent wetland basins to extensively culture walleye Sander vitreus fry. Waterfowl managers have expressed concern over this practice because of the potential influence that fish have on food resources used by waterfowl during development and migration. It is well known that native fathead minnows Pimephales promelas can have detrimental effects on macroinvertebrates, zooplankton, water clarity, epiphyton, and macrophytes in wetlands. Because walleye commonly become piscivorous as soon as mouth gape allows, walleye fry may suppress fathead minnow populations and improve wetland conditions for waterfowl. In this study, we quantify consumption estimates, specifically predation on fathead minnows, by age‐0 and age‐1 walleye reared in natural wetland basins. Six wetlands were stocked in mid‐May 2001 and 2002 at a rate of 12,000 walleye fry ha–1. Age‐0 walleye were sampled bi‐weekly from mid‐June through mid‐September 2001. Age‐0 and age‐1 walleye were sampled monthly from mid‐May through mid‐September 2002. A generalised diet shift from zooplankton to fish to macroinvertebrates was observed in 2001, whereas diets of juvenile walleye contained primarily macroinvertebrates in 2002. Stocked walleye quickly reduced fathead minnow populations in 2001 and suppression was maintained throughout 2002. Although walleye consumed primarily macroinvertebrates once prey fish populations became suppressed, consumption estimates of invertebrates by walleye were substantially less than those documented for fathead minnow populations. Thus, stocking age‐0 walleye was an effective biomanipulation tool that substantially reduced fathead minnow densities and influenced lower trophic levels in these aquatic communities.

Population ecology and prey consumption by fathead minnows in prairie wetlands: importance of detritus and larval fish

Abstract – The fathead minnow Pimephales promelas occurs in high densities in wetlands of the prairie pothole region (PPR) of North America, but food resources sustaining these populations are poorly known. We assessed population dynamics and prey consumption of fathead minnow populations in three PPR wetlands for 2 years. Fish density peaked at 107 fish per m2 for all age classes combined. Larval and juvenile fish dominated these populations in terms of abundance and accounted for 83% of total prey consumption. Detritus dominated fish diets, representing 53%, 40% and 79% of diet mass for larval, juvenile and adult fish respectively. Detritus consumption was positively related to minnow density and negatively related to invertebrate abundance, but only for adult fish. Seasonal production:biomass ratios were unrelated to proportions of detritus in the diet for all ages of fish, indicating that detritus is an important food resource capable of meeting metabolic demands and sustaining fish growth in high‐density populations. Detritus consumption may also weaken links between abundance of invertebrate prey and minnows, promoting dense fish populations with strong, consistent influences on wetland ecosystems.

Linkage of biochemical responses to population‐level effects: A case study with vitellogenin in the fathead minnow (Pimephales promelas)

A challenge in the field of ecotoxicology is the linkage of alterations at molecular and biochemical levels of organization to adverse outcomes in individuals and populations. In the present study, a predictive relationship between plasma vitellogenin (VTG) concentration and fecundity in female fathead minnows (Pimephales promelas) was derived from 21-d laboratory toxicity tests with five chemicals (17beta-trenbolone, 17alpha-trenbolone, prochloraz, fenarimol, and fadrozole) that inhibit VTG production through different mechanisms. Because VTG is key to egg production in female oviparous animals, changes in the lipoprotein could, theoretically, serve as an indicator of reproductive success. Regression of fecundity versus VTG concentration from the various studies yielded a highly significant linear model (fecundity = -0.042 + 0.95 x VTG, p < 0.01, r2 = 0.88). This relationship was integrated into a population model to translate changes in VTG concentrations of female fathead minnows to alterations in population growth. The model predicted relatively profound effects on population size of fish experiencing moderate decreases in vitellogenesis. For example, a fathead minnow population at a carrying capacity exposed to a chemical stressor that causes a 25% decrease in VTG concentration in females from baseline values would exhibit a 34.6% projected decrease in size after two years of exposure and reach an equilibrium population size that was only 30.2% of the preexposed population. Overall, the current study provides an example of how changes in a biomarker (VTG concentration) can be quantitatively translated into adverse effects at the individual and population levels.

Evaluation of acute copper toxicity to larval fathead minnows (Pimephales promelas) in soft surface waters

The hardness-based regulatory approach for Cu prescribes an extrapolation of the toxicity-versus-hardness relationship to low hardness (< or =50 mg/L as CaCO3). Hence, the objective of the present research was to evaluate the influences of water quality on acute Cu toxicity to larval fathead minnow (Pimephales promelas) in low-hardness surface waters. Seasonal water sampling was conducted at 24 sites throughout South Carolina, USA, to determine the site-specific influences of soft surface-water conditions on acute Cu toxicity. Concurrent toxicity tests in laboratory water, matched for hardness and alkalinity (modified method), also were conducted to allow calculation of water-effect ratios (WERs). In addition, tests were conducted at recommended hardness levels (recommended method) for comparison of WER methodology in soft water. Surface-water conditions (average+/-standard deviation, n = 53) were hardness of 16+/-8 mg/L as CaCO3, alkalinity of 18+/-11 mg/L as CaCO3, and dissolved organic carbon of 6+/-4 mg/L. Dissolved Cu 48-h median lethal concentration (LC50) values varied nearly 45-fold across the dataset and greater than four-fold at individual sites. Spatial (p < 0.0001) and seasonal (p = 0.026) differences among LC50 values were determined for eight sites that had multiple toxicity results for one year. All modified WERs were greater than 1.0, suggesting that the site waters were more protective of Cu toxicity than the matched laboratory water. Some WERs generated using recommended methods were less than 1.0, suggesting limited site-specific protection. Based on these observations, extrapolation of the hardness-based equation for Cu at 50 mg/L or less as CaCO3 would adequately protect fathead minnow populations in soft surface waters. The WER results presented here demonstrate the inconsistency between hardness-based criteria and the methodology for deriving site-specific water-quality criteria in low-hardness waters.

Modeling impacts on populations: fathead minnow ( Pimephales promelas) exposure to the endocrine disruptor 17 β-trenbolone as a case study

Evaluation of population-level impacts is critical to credible ecological risk assessments. In this study, a predictive model was developed to translate changes in fecundity of the fathead minnow ( Pimephales promelas) in a short-term laboratory toxicity test to alterations in population growth rate. The model uniquely combines a Leslie population projection matrix and the logistic equation. Application of the model requires only a life table for the organism of interest, a measure of carrying capacity for the given population, and an estimation of the effect of a stressor on vital rates. The model was applied to investigate population dynamics for fathead minnow exposed to the androgen receptor agonist 17 β-trenbolone. Organismal-level responses for fathead minnows exposed to varying levels of 17 β-trenbolone were used to determine projected alterations in a population existing in a small body of water containing varying concentrations of the androgen. Fathead minnow populations occurring at carrying capacity and subsequently exposed to 0.027 μg/L of 17 β-trenbolone exhibited a 51% projected decrease in average population size after 2 years of exposure. Populations at carrying capacity exposed to concentrations of 17 β-trenbolone ⩾0.266 μg/L exhibited a 93% projected decrease in average population size after 2 years of exposure. Overall, fathead minnow populations exposed to continued concentrations of 17 β-trenbolone equal to or greater than 0.027 μg/L were projected to have average equilibrium population sizes that approached zero.